What gravitational waves can reveal about dark matter?
According to scientists,
dark matter might occasionally produce gravitational waves strong enough to be picked up by instruments like LIGO.
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| Photo credit- Wikimedia commons |
Albert Einstein's theory of general relativity, which was published in 1916, created the contemporary understanding of gravity as a warping of spacetime. According to the hypothesis, everything that interacts with gravity could cause ripples in that fabric.
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| Photo credit- National Science Foundation |
Gravitational waves can be generated by any object that interacts with gravity. However, only the most catastrophic cosmic events produce gravitational waves strong enough for us to observe. Now that observatories have begun to record gravitational waves on a regular basis, scientists are debating how dark matter, which has only been observed to interact with other matter through gravity, may produce gravitational waves powerful enough to be detected.
The spacetime blanket
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| Photo credit- Perimeter Institute |
Each of these objects adds weight to the area of the blanket where it is placed, and the heavier the object, the deeper the depression in the blanket. The fabric of spacetime is affected by objects of various masses in ways similar to how different weights affect a blanket. The gravitational field causes depression in spacetime.
One object's gravitational field can have an impact on another object's field. The second item may enter the gravitational field of the first and orbit it, just like the moon orbits the Earth or the Earth orbits the sun.
Alternatively, two bodies with gravitationally charged objects will spiral toward one another and eventually crash. Gravitational waves are produced as a result, which is spacetime ripples.
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| Photo credit - Wikipedia |
On September 14, 2015, scientists used the Laser Interferometer Gravitational-Wave Observatory (LIGO) to make the first direct observations of gravitational waves that are part of an accumulation of collisions between two massive black holes.
Since that initial discovery, the LIGO team—along with the cooperation that operates a companion gravitational-wave observatory named Virgo—has discovered gravitational waves from at least 10 other black hole mergers, as well as the first merger of two neutron stars in 2017.
It's estimated that dark matter predominates five times more often than visible matter. The entire cosmos is affected by its gravitational effects. Although there are many possibilities, scientists believe they have not yet observed gravitational waves that are directly related to dark matter.
Primordial black holes
Dark matter must exist since scientists have observed its gravitational effects, or at the very least, something must be happening to produce those effects. But since they haven't yet actually observed a dark matter particle, they are unsure of the specific characteristics of dark matter.
One hypothesis is that a portion of the dark matter may actually be primordial black holes.
Imagine the entire cosmos as a gigantic petri dish. In this scenario, the Big Bang is the point where matter-bacteria begins to grow. The petri dish gradually enlarges as that spot quickly moves outward and grows larger. Certain regions will develop into denser regions of matter than others if that growth is slightly unequal.
These clusters of concentrated matter, which are primarily photons at this stage of the cosmos, may have collapsed under the force of their own gravity to create the first black holes.
“I think it’s an interesting theory, as interesting as a new kind of particle,” says Yacine Ali-Haimoud, an assistant professor of physics at New York University. “If primordial black holes do exist, it would have profound implications on the conditions in the very early universe.”
It is possible that LIGO will be able to prove or constrain this dark matter theory by using gravitational waves to learn more about the characteristics of black holes.
Primordial black holes don't need to achieve a minimum mass threshold in order to form, in contrast to conventional black holes. For instance, if LIGO discovered a black hole with a mass smaller than the sun, it might be a primordial black hole.
It's unlikely that primordial black holes account for all of the dark matter in the cosmos, even if they do exist. Nevertheless, confirming the existence of primordial black holes would advance our fundamental knowledge of dark matter and the origin of the universe.
Neutron star rattles
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| Photo credit - NASA/CXC/Trinity University/ D.Pooley et al. |
According to the known interactions of particles, dark matter may interact with itself as well as normal matter, even though it only appears to interact with normal matter through gravity.
If so, it's possible that dark matter particles will combine to create objects that are nearly as dense as neutron stars.
We are aware that stars significantly "weigh down" the spacetime around them. If the universe were populated with compact dark objects, there would be a chance that at least some of them would end up trapped inside of ordinary matter stars.
We are aware that stars significantly "weigh down" the spacetime around them. There would be a possibility that at least some compact dark objects might become caught inside of ordinary matter stars if the universe were filled with them.
A regular star and a dark object would only interact through gravity, making it possible for the two to coexist peacefully. However, any alteration to the star, like as a supernova explosion, could cause a rattle-like disturbance between the emerging neutron star and the trapped dark object. There would be detectable gravitational waves if such an occurrence took place in our galaxy.
“We understand neutron stars quite well,” says Sanjay Reddy, University of Washington physics professor and senior fellow with the Institute for Nuclear Theory. “If something ‘odd’ happens with gravitational waves, we would know there was potentially something new going on that might involve dark matter.”
In our solar system, there is a slight chance that any may exist. A recent analysis of LIGO's data by Chuck Horowitz, Maria Alessandra Papa, and Reddy revealed no evidence of compact dark objects within Earth, Jupiter, or the sun that fall within a particular mass range.
Additional gravitational-wave research may impose additional constraints on compact dark objects. “Constraints are important,” says Ann Nelson, a physics professor at the University of Washington. “They allow us to improve existing theories and even formulate new ones.”
Axion stars
One light-dark matter candidate is the axion, named by physicist Frank Wilczek after a brand of detergent, in the sense that it can solve a difficulty in the quantum chromodynamics theory.
Scientists believe that axions could combine to form axion stars. Axion stars are similar to neutron stars, but consist of very compact axion matter.
“If axions exist, there are scenarios where they can cluster together and form stellar objects, like ordinary matter,” says Tim Dietrich, a LIGO-Virgo member, and physicist. “We don’t know if axion stars exist, and we won’t know for sure until we find constraints for our models.”
When an axion star merges with a neutron star, scientists using current instruments may not be able to tell the difference between the two stars. Instead, scientists must rely on the electromagnetic signals that accompany gravitational waves to identify anomalies.
Axions may also cluster around binary black holes and neutron star systems. When these stars merge, changes in the axion 'cloud' appear in the gravitational wave signal. A third possibility is that an axion can result from a merge, an action reflected in a signal.
This month, the LIGO-Virgo collaboration will begin observations for the third time, with new upgrades expected to discover one fusion event each week.
Gravitational wave detectors have already proven successful in confirming Einstein's centuries-old predictions. But there is much more that gravitational wave research can tell us. “Gravitational waves are like a completely new sense for science,” Ali-Haimoud says. “A new sense means new ways to look at all the big questions in physics.”
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